Durability of GFRP Reinforcing Bars Embedded in … of GFRP Reinforcing Bars Embedded in Moist...

Durability of GFRP Reinforcing Bars Embeddedin Moist Concrete

Mathieu Robert1; Patrice Cousin2; and Brahim Benmokrane3

Abstract: This paper presents mechanical, microstructural, and physical characterization of glass fiber-reinforced polymer �GFRP� barsexposed to concrete environment. GFRP bars were embedded in concrete and exposed to tap water at 23, 40, and 50°C to accelerate theeffect of the concrete environment. The measured tensile strengths of the bars before and after exposure were considered as a measure ofthe durability performance of the specimens and were used for long-term properties prediction based on the Arrhenius theory. In addition,Fourier transform infrared spectroscopy, differential scanning calorimetry, and scanning electron microscopy were used to characterize theaging effect on the GFRP reinforcing bars. The results showed that the durability of mortar-wrapped GFRP bars and exposed to tap waterwas less affected by accelerated aging than the bars exposed to simulated pore-water solution. These results confirmed that the concernsabout the durability of GFRP bars in concrete, based on simulated laboratory studies in alkaline solutions, do not properly correspond tothe actual service life in concrete environments.

DOI: 10.1061/�ASCE�1090-0268�2009�13:2�66�

CE Database subject headings: Durability; Fiber reinforced polymers; Moisture; Aging; Alkalinity; Predictions; Mechanicalproperties; Microstructures.

Introduction

Fiber-reinforced polymers �FRP� have been widely used in aero-nautical and chemical engineering for decades. FRP materials arealso increasingly being used for engineering applications. How-ever, the low cost-to-performance advantage of glass fiber re-inforced polymers �GFRP� is the driving force behind itsworldwide use and acceptance. GFRP materials are attributed tobeing of high strength, light weight, noncorrosive, and noncon-ductive. Unfortunately, in some special conditions, such as in ahigh alkalinity environment, the long-term performance of theGFRP is still an unresolved question. Strength of the glass fibersand resin matrix, the two constituents of the GFRP materials, candecrease when subjected to a wet alkaline environment.

Several research works were carried out to investigate thedurability of GFRP materials under different environmentalconditions that are anticipated under actual service conditions.Sen et al. �1999� investigated the durability of S-2 glass/epoxyprestressed beams exposed to wet/dry cycles in a 15% saltsolution and found that GFRP bars lost their effectiveness within

1Ph.D. Candidate, Dept. of Civil Engineering, Univ. of Sherbrooke,Sherbrooke PQ, Canada J1K 2R1. E-mail: [email protected]

2Research Associate, Dept. of Civil Engineering, Univ. of Sherbrooke,Sherbrooke PQ, Canada J1K 2R1. E-mail: [email protected]

3NSERC Research Chair Professor in Innovative FRP CompositeMaterials for Infrastructures, Dept. of Civil Engineering, Univ. ofSherbrooke, Sherbrooke PQ, Canada J1K 2R1 �corresponding author�.E-mail: [email protected]

Note. Discussion open until September 1, 2009. Separate discussionsmust be submitted for individual papers. The manuscript for this paperwas submitted for review and possible publication on October 4, 2007;approved on November 25, 2008. This paper is part of the Journal ofComposites for Construction, Vol. 13, No. 2, April 1, 2009. ©ASCE,
ISSN 1090-0268/2009/2-66–73/$25.00.
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3–9 months of exposure. S-2 glass has many of the features ofglass fiber with increased strength and temperature resistance.Porter et al. �1997� exposed three different types of E-glass FRPbars �manufactured using an isophthalic polyester resin� to highalkaline solution and a maximum temperature of 60°C for a pe-riod of 2–3 months. Their test results indicated that the acceler-ated aging severely reduced the ultimate tensile strength and themaximum strain capacity of the GFRP bars. Dejke �1999� re-ported that glass fibers are known to degrade in the presence ofwater, and that moisture can decrease the glass transition tempera-ture �Tg� of the resin and act as a plasticizer, potentially havingsignificant effect on flexural strength. The reaction of FRPcomposite with alkali of concrete is one of the major durabilityconcerns for design engineers. Typically, concrete environmenthas high alkalinity, which depends on the design mixture of theconcrete, and the type of cement used �Diamond 1981; Taylor1987�. This alkaline environment damages glass fibers throughloss in toughness, strength, and embrittlement. Recently, Chen etal. �2007� studied the durability of bare FRP bars and bars em-bedded in concrete immersed in different solutions at differenttemperatures and time of exposure. The writers found that GFRPbare bars and embedded in concrete bars showed a significantstrength loss when exposed to simulated environments, especiallyfor solutions at high temperature of 60°C �Chen et al. 2007�.

Glass fibers are damaged due to the combination of two pro-cesses: �1� chemical attack on the glass fibers by the alkalinecement environment; and �2� concentration and growth of hydra-tion products between individual filaments �Murphy et al. 1999�.The embrittlement of fibers is due to the nucleation of calciumhydroxide on the fiber surface. The hydroxylation can cause fibersurface pitting and roughness, which act as flaws severely re-ducing fiber properties in the presence of moisture. In addition,calcium, sodium, and potassium hydroxides found in the concretepore solution are aggressive toward glass fibers �Benmokrane
et al. 2002, Nkurunziza 2004�. Therefore, the degradation of glass
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fibers not only depends on the high pH level, but also on thecombination of alkali salts, pH, and moisture.

Aqueous solutions with high pH are known to degrade thetensile strength of GFRP bars �Chen et al. 2006�, although par-ticular results strongly vary according to differences in test meth-ods. Higher temperature and long exposure time exasperate theproblem. Most of the data were generated using temperatures aslow as slightly below the freezing point and as high as a fewdegrees below the glass transition temperature of the resin. Ten-sile strength reductions in GFRP bars ranging from 0 to 75% ofinitial values have been reported in the literature cited �Katsukiand Uomoto 1995; Dejke 1999; Chen et al. 2006�. Hence, it isclear that FRP material components and test methods have animportant influence on the conclusions regarding the durability ofFRP bars in concrete �Mufti et al. 2007�.

In order to evaluate long-term durability performance of FRPin an alkaline environment, extensive studies have been con-ducted to develop accelerated aging procedures and predictivemodels for long-term strength estimates, especially for GFRP bars�Porter et al. 1997; Dejke 2001; Bank et al. 2003; Chen et al.2006�. These models are based on an Arrhenius-type model pro-posed by Litherland and his colleagues �Litherland et al. 1981�.Research studies on the effects of temperature on the durability ofFRP bars in concrete alkaline environment indicates that an ac-celerated factor for each temperature difference can be defined byusing Arrhenius laws. These factors differ for each product, de-pending on the types of fiber and resin and bar size. In addition,the factors are affected by the environmental conditions, such assurrounding solution media, temperature, pH, moisture, andfreeze-thaw conditions. Predictive models based on Arrheniuslaws make the implicit assumption that the elevated temperaturewill only increase the rate of degradation without affecting thedegradation mechanism or introducing other degradation mecha-nisms of FRP bars. Gerritse �1998� indicated that at least threeelevated temperatures are necessary to perform an accurate predi-cation based on Arrhenius law. Moreover, the measured datashould be in continuous time intervals.

Until recently, experts were not in full agreement aboutwhether or not GFRP is stable in the alkaline environment ofconcrete. In a recent study �Mufti et al. 2007�, nine cores weretaken from each of five in-service concrete bridge structuresacross Canada, reinforced with GFRP bars; these structures werebuilt during the last 6–8 years. Three cores from each bridgewere given to each of three teams of material scientists and ex-perts in durability, for microscopic and chemical analyses. Anexample from this study is presented in Fig. 1, which shows amicrograph of GFRP and surrounding concrete removed from an8-year-old structure. It can be seen that the glass fibers and theGFRP/concrete interface are intact. The findings from the analy-ses reported by Mufti et al. �2007� have confirmed that the con-cerns about the durability of GFRP in alkaline concrete, based onsimulated laboratory studies in alkaline solutions, are unfounded.It is mainly on the basis of this study that GFRP is now permittedby the Canadian highway bridge design code �CSA 2006� as bothprimary reinforcement and prestressing tendons in concrete�Mufti et al. 2007�.

The current study is aimed to simulate and approach real con-ditions by immersing mortar-wrapped GFRP bars in tap water.The conditioning used in the present study is closer to field con-ditions because the FRP material is embedded in concrete whichis the actual situation in the field. Indeed, the main objective ofthis study is to show that the traditional accelerated aging in al-
kaline solution is too severe and prematurely degrades the GFRP
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reinforcing bars, leading to limited life expectancies and too con-servative predictions.

Experimental Program

Material

Sand coated glass FRP bars manufactured by a Canadian com-pany �Pultrall Inc. 2005� were used in this study. The bars weremade of continuous E glass fibers impregnated in a vinylesterresin using the pultrusion process. The mass fraction of glass is81.5% and was determined by thermogravimetric analysis accord-ing to ASTM E 1131 �ASTM 2003b� standard. Their relativedensity according to ASTM D 792 standard is 1.99 �ASTM2000�. The mechanical and physical properties of 12.7 mm diam-eter GFRP bars, as provided by the manufacturer, are summarizedin Table 1. The GFRP bars used have a nominal diameter of12.7 mm. All bars were cut into 1,440 mm lengths as specified bythe ACI 440.3R-04 B2. The bars were divided into two series: �1�the unconditioned reference samples; and �2� conditioned samples�60 bars� embedded in concrete. The mortar mixture consisted ofthree parts of sand, one part of Type I cement according to ASTMC 150 �ASTM 2005� standard, and a water/cement ratio of 0.40which led to a concrete pH of 12.15 measured by extraction ofthe interstitial solution after aging. The concrete �or mortar� wascast only in the middle third of the bars to avoid any degradationat the ends which were used as grips during the tensile testsACI440.3R-04 B2. The concrete mold of the envelope was madeof wood having a square section of 48 mm that leads to a mini-mum concrete cover of 18 mm. Fig. 2 shows a sketch illustratinga mortar-wrapped bar, whereas Fig. 3 shows a picture of a mortar-wrapped bar.

Test Plan

Accelerated aging of GFRP reinforcing bars embedded in con-crete, used in this study, were designed to simulate an aggressivealkaline environment of saturated concrete. The embeddedsamples were immersed in tap water which differs from the con-ventional accelerated aging tests where bare bars are directly im-

Fig. 1. Micrograph of GFRP bar/concrete interface in specimenremoved from 8-year-old GFRP reinforced concrete structure �Muftiet al. 2007�

mersed in alkaline solutions simulating the aforementioned

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results. The technique currently used is believed to be more rep-resentative of the real life situation. Indeed, the pH of the solutionsurrounding the bars is a result of the absorption of water by theconcrete, thus allowing the liberation of the alkaline ions of theconcrete directly in the bars environment. The aging was per-formed by immersing the mortar-wrapped GFRP bars in a woodcontainer specially manufactured for the study. Fig. 4 shows thecontainers that were tightly closed with a polyethylene film ontheir inner surfaces. A polyethylene sheet was also placed on topof the wood containers to avoid excessive evaporation of waterduring the conditioning. Bars were spaced from each other andfrom the bottom of the container to allow free circulation of thesolution between and around the GFRP bars. Furthermore, thewater level was kept constant throughout the study to avoid a pHincrease which could be due to a water level decrease and a

Table 1. Mechanical and Physical Properties of 12.7 mm Diameter GFR

Property

Mechanical properties Nominal tensile str

Guaranteed design tensi

Nominal tensile mo

Tensile strain

Poisson’s ratio

Nominal flexural st

Nominal flexural m

Flexural strain

Nominal compressive

Nominal shear str

Physical properties Longitudinal coefficient of th

Transverse coefficient of the

Moisture absorp

Glass content

Fig. 2. Sketch illustrating typical cement mortar-wrapped GFRP barspecimen

Fig. 3. View of cement mortar-wrapped GFRP bar specimen



significant increase of the concentration of the alkaline ions in thesolution. The temperatures of immersion were chosen to acceler-ate the degradation effect of aging; however, they were not toohigh to produce any thermal degradation mechanisms.

The specimens were completely immersed at three differenttemperatures �23, 40, and 50°C� and were removed from thewater after four different periods of time �60, 120, 180, and240 days�. After each period, usually six GFRP bars were re-moved from the water and tested under tension to compare theirtensile strength retention values to those of the reference speci-mens. After immersion, no significant change was observed at thesurface of GFRP bars.

Tensile Tests

All bars were tested under tension according to the ACI440.3R-04 B2. Each specimen was instrumented with a linearvariable differential transformer �LVDT� to capture the elongationduring testing. The test was carried out using a Baldwin testingmachine and the load was increased until failure. For each tensiletest, the specimen was mounted on the press with the steel pipeanchors gripped by the wedges of the upper and the lower jaw ofthe machine. To avoid any damage to the bar, the concrete cover

�as Provided by Manufacturer�

Unity Value

MPa 786

ngth MPa 708

GPa 46.3

% 1.70

— 0.26

MPa 1,005

GPa 46.8

% 2.15

th MPa 473

MPa 212

expansion �10−6 /°C 5.5

xpansion �10−6 /°C 29.5

% 0.38

% volume 60.3

% by weight 77.9

Fig. 4. Wood container built for aging of cement mortar-wrappedGFRP bar specimens

P Bar

ength

le stre

dulus

rength

odulus

streng

ength

ermal

rmal e

tion

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was carefully removed from the middle third of the specimenswith a hammer. The rate of loading ranged between 250 and500 MPa /min. The applied load and bar elongation were re-corded during the test using a data acquisition system monitoredby a computer. Due to the brittle nature of GFRP, no yieldingoccurs and the stress-strain behavior was linear.

Arrhenius Relation

Eq. �1� expresses the Arrhenius relation, in terms of the degrada-tion rate �Nelson 1990�

k = A exp�− Ea

RT� �1�

where k=degradation rate �L/time�; A=constant relative to thematerial and degradation process; Ea=activation energy of thereaction; R=universal gas constant; and T=temperature �K�. Theprimary assumption of this model is that only one dominant deg-radation mechanism of the material operates during the reactionand that this mechanism will not change with time and tempera-ture during the exposure �Chen et al. 2006�. Only the rate ofdegradation will be accelerated with the temperature increase. Eq.�1� can be transformed into

1

k=

1

Aexp� Ea

RT� �2�

ln�1

k� =

Ea

R

1

T− ln�A� �3�

From Eq. �2�, the degradation rate k can be expressed as theinverse of time needed for a material property to reach a givenvalue �Chen et al. 2006�. From Eq. �3� one can further observethat the logarithm of time needed for a material property to reacha given value is a linear function of 1 /T with the slope of Ea /R�Chen et al. 2006�. Ea and A can be easily calculated by using theslope of the regression and the point of intersection between theregression and the Y axis, respectively.

Scanning Electron Microscopy

Scanning electron microscopy �SEM� observations and imageanalysis were performed to observe the microstructure of speci-mens before and after aging. Samples observed in the SEM werethe unconditioned specimens and specimens aged during8 months in tap water at 50°C, which is a harsher aging. Allspecimens observed in the SEM were first cut, polished, andcoated with a thin layer of gold-palladium by a vapor-depositprocess. After coating the surfaces, microstructural observationswere performed on a JEOL JSM-840A SEM. These observationswere conducted to see the potential degradation of polymer ma-trix, glass fibers, or interfaces, if any.

Differential Scanning Calorimetry

Twelve–15 mg specimens from both unconditioned and agedsamples were sealed in aluminum pans and analyzed in a TAInstruments DSC Q10 calorimeter equipped with a refrigeratedcooling system. Analysis was conducted in modulated differentialscanning calorimetry �DSC� mode. Specimens were heated from25 to 195°C at a rate of 5°C /min. Glass transition temperaturewas determined for both specimens in accordance with ASTM E
1356 standard �ASTM 2003a�. Two scans were performed for
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each specimen. The first scan is useful to determine the differenceof Tg between reference and conditioned specimens. If a decreaseof Tg is observed for conditioned samples, this is an indication ofplasticizing effect or chemical degradation. The second scan givesinformation about the mechanism of degradation. A shift forhigher Tg could be observed due to a post cure during the firstscan. However, if after the second scan, the Tg of aged sample isin the same range as the Tg of reference sample, this is a revers-ible plasticizing effect due to the moisture absorption, but if theaged sample keeps a Tg lower than for the reference, this is airreversible chemical degradation.

Fourier Transform Infrared Spectroscopy

Fourier transform infrared spectroscopy �FTIR� spectra were re-corded using a Nicolet Magna-550 spectrometer equipped with aattenuated total reflectance �ATR� device. Fifty scans were rou-tinely acquired with an optical retardation of 0.25 cm to yield aresolution of 4 cm−1.

Tests Results and Discussion

Tensile Strength Retention

The tensile test of unconditioned specimens showed an approxi-mately linear behavior up to failure. Specimens failed through therupture of fibers. The failure was accompanied by the delamina-tion of fibers and resin, as shown in Fig. 5. A similar, but lesscatastrophic failure, was observed for specimens embedded inconcrete and immersed in water. No chemical deposit was ob-served on the surface of the bars before testing. Micelli and Nanni�2004� also observed similar tensile failure modes of GFRP bars.

Table 2 shows the experimental results obtained during thetensile tests concerning the ultimate strength of aged bars testedafter immersion. Fig. 6 shows the retention of the ultimatestrength of aged bars according to the duration of immersion ofembedded bars at various temperatures. As shown in Table 2, thetensile strength for the unconditioned bar was equal to788�54 MPa. Note that the tensile strength of bars was reducedto 665�62 MPa after 240 days exposure to water at 50°C. Also,the result obtained after a conditioning of 120 days at 40°C was

Fig. 5. Typical failure mode of conditioned GFRP bar specimenssubjected to tensile test

rejected for long-term prediction, because the mode of failure



observed during the tensile test was different than those observedby testing conditioned samples at different temperatures.

Fig. 6 shows a slight decrease of the ultimate tensile strengthwith immersion duration. The recorded results show that thelonger the time of immersion, the larger the loss of resistance.Furthermore, it is clear that the temperature of immersion affectsthe loss of resistance. It can be seen that for duration of immer-sion of 8 months, the loss of resistance is equal to 16, 10, and 9%at 50, 40, and 23°C, respectively. This phenomenon is due to theincrease of the diffusion rate of the solution and to the accelera-tion of the chemicals reaction of degradation with the temperatureof immersion, leading to a larger absorption rate of the solutionfor the same time of immersion and accelerated reaction of deg-radation. The absorption of solution can lead to a degradation ofthe fibers and fiber/matrix interface, leading to a loss of the ulti-

Table 2. Experimental Tensile Strength of Reference and ConditionedSpecimens

Time ofimmersion�days�

Temperature�°C�

Meantensile

strength�MPa�

Coefficientof variation

0 23 788 0.069

23 753 0.082

60 40 755 0.037

50 767 0.038

23 702 0.020

120 40 666 0.078

50 720 0.032

23 717 0.026

180 40 708 0.048

50 711 0.025

23 714 0.035

240 40 708 0.071

50 665 0.093

Fig. 6. Tensile strength retention of conditioned GFRP bars aged inmoist concrete at 23, 40, and 50°C



mate tensile. In a similar study, Wang �2005� recorded losses ofresistance of 51, 25, and 32% after 300 days aging in alkalinesolution �pH 12.8� of the same bars at 60, 40, and 23°C, respec-tively. So, for a similar period of exposure in the solutions of thesame pH value, the losses of tensile strength of the GFRP barsaged in alkaline solution were more than three times larger thanthose of the GFRP bars aged in moist concrete.

Young’s Modulus

Fig. 7 shows the change in the elastic modulus of aged bars withtime of immersion at various temperatures. Indeed, it can be seenfrom the measured results that after 240 days, the loss of elasticmodulus is negligible and all aged bars are not affected by thehigher temperature or the exposure to moist concrete. This resultshows that elastic modulus of bars is not affected by aging in aconcrete environment.

Microstructural Effects

The visual and microstructural observations showed no significantdamage after 240 days of immersion in tap water at the highesttemperature �50°C�. The micrographs of Fig. 8 show the inter-

Fig. 7. Elastic modulus retention of conditioned GFRP bars aged inmoist concrete at 23, 40, and 50°C

a) b)

Fig. 8. Micrograph �X20� of: �a� unconditioned bar; �b� mortar-wrapped FRP bar aged in water at 50°C for 240 days

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face between the mixture of fibers and resin and the silica coatingof the GFRP bar for an unconditioned bar and for a mortar-wrapped bar and aged in tap water for 240 days. Fig. 9 showsmicrographs of the fibers/matrix interface for the same sample.

Observation of these interfaces and of the microstructure, ingeneral, demonstrate that the conditionings of mortar-wrappedbars in water do not affect the microstructural properties of theGFRP bars; this contradicts what several researchers had notedearlier when similar GFRP bars were directly immersed into analkaline solution �Benmokrane and Wang 2002�. This phenom-enon illustrates the fact that GFRP bars are not significantly af-fected by accelerated aging.

This observation proposes that the standard accelerated agingin alkaline solution could very severely alter the properties ofaged bars compared to tap water aging which could to be a bettersimulation of the real service.

a) b)

Fig. 9. Micrographs of fiber/matrix interface �X3000� of: �a� uncon-ditioned GFRP bar specimen; �b� conditioned GFRP bar specimenaged in moist concrete at 50°C for 240 days

(a) Unconditioned GFRP bar specimen at

low magnification

(b) Surface of GFRP bar specimen aged in

moist concrete at 50oC for 240 days at low

magnification

(c) Surface of GFRP bar specimen aged in

moist concrete at 50oC for 240 days at high

magnification

(d) GFRP bar specimen aged in alkaline

solution at 60oC for 300 days at high

magnification (Wang, 2005)

Fig. 10. Comparison of micrographs of external surfaces of GFRPbar specimens aged in moist concrete �present study� and in alkalinesolution �Wang 2005�

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Micrographs of External Surfaces of GFRP BarThe micrographs of external surfaces of GFRP bar specimensaged in moist concrete �present study� and in alkaline solution�Wang 2005� are shown in Fig. 10. It should be noted that inhis durability tests Wang �2005� has used the same GFRP bar aswas used in the present study. The comparison of these micro-graphs shows that there was no significant damage that occurredto the mortar-wrapped GFRP bar surface �Figs. 10�b and c��,whereas the damage was clear on the bar aged in an alkalinesolution �Fig. 10�d��. Moreover, the GFRP bar aged in alkalinesolution shows a porous surface due to its direct contact with thesolution �Fig. 10�d��.

GFRP Bar/Concrete InterfaceThe three main components of the bond between the GFRP rein-forcing bar and concrete are: �1� the chemical adhesion; �2� themechanical interlocks; and �3� the friction. Therefore, if there isany degradation at the resin or the fibers at the interface, it isexpected that those mechanisms and the durability of GFRP rein-forced structure could be affected. Fig. 11 shows the GFRP bar/concrete interface for the mortar-wrapped GFRP bar and aged inwater at 50°C for 240 days. Again, it can be seen that there wasno significant damage to the interface between the GFRP bar andconcrete after aging in tap water.

Effects on Matrix

A FTIR analysis of unconditioned and 12.7 mm mortar-wrappedbars and aged in water at 50°C for 240 days has been conducted�Fig. 12�. The most interesting region of the FTIR spectra is lo-cated between 3,300 and 3,600 cm−1, which corresponds to thestretching mode of the hydroxyl groups of the vinylester resin.

(a) Unconditioned GFRP bar (b) Conditioned GFRP bar aged in moist

concrete at 50oC for 240 days

1 mm 1 mm

Fig. 11. Micrographs of GFRP bar/concrete interface of: �a� uncon-ditioned bar; �b� conditioned GFRP bar aged in moist concrete at50°C for 240 days

250027002900310033003500370039004100wavelength (cm-1)

Reference

Immersed 8 monthsin water at 50oC

Absorpbance

Fig. 12. FTIR spectra for unconditioned and aged samples



When a hydrolysis reaction occurs, new hydroxyl groups areformed and the corresponding infrared band increases. Changes inthe peak intensity were quantified by determining the ratio of theOH peak to the carbon–hydrogen stretching peak of the resin,which is not affected by the conditioning. The experimental ratioof the OH peak to the carbon–hydrogen stretching peak of theresin for the 12.7 mm diameter mortar-wrapped samples and im-mersed in water for 240 days at 50°C was 0.25 compared to 0.21for unconditioned samples. Therefore, the hydroxyl peak does notshow any significant changes. This indicates that the hydrolysisdid not significantly occur in these environmental conditions.Wang �2005� obtained changes in ratio up to 30% when sampleswere immersed in alkaline solution at 60°C for 300 days. Theseresults show that alkaline solutions are too harsh as compared tothe immersion of the mortar-wrapped GFRP bar in water, whichbetter simulate the real application environment.

Table 3 presents the glass transition temperature �Tg� valuesfor the first and second heating unconditioned and aged samples.Note that for the unconditioned and aged samples, the Tg corre-sponding to the second heating run is higher than that of the firstscan. This shift indicates that the samples were not fully curedand that a postcuring phenomenon occurred during the secondheating run. However, it can be seen from the results presented inTable 3 that no significant changes of the Tg value occur afteraging in water at 50°C for 240 days. This result shows that nomajor effect on the thermal properties of the resin, which couldoccur after conditioning of mortar-wrapped FRP bars, was de-tected by DSC.

Prediction of Long-Term Behavior

Following the procedure proposed by Bank et al. �2003�, the natu-ral logarithm of time to reach a set of levels of normalized per-formances versus 1 /T, expressed as the inverse of absolutetemperature �1,000 /K�, was used to predict the service life at themean annual temperature �6°C� in Montréal. A coefficient of de-termination �R2� value close to 1 is desired. However, the ASTMprocedures recommend a minimum value of 0.80 for acceptabilityand the obtained R2 values are between 0.94 and 0.99. Fromthe Arrhenius plot, the service lifetime necessary to reach theestablished tensile strength retention levels �PR� can be extrapo-lated for any temperature. Consequently, predictions are madefor tensile strength retention as a function of time for an immer-sion at 6°C and the general relation between the PR and thepredicted service life at the average temperature of 6°C aredrawn �Fig. 13�. The predicted time to reach the determinedstrength property retention level �PR� for GFRP bars embedded inmoist concrete at an isotherm temperature of 6°C is approxi-mately 1 and 210 years for a PR of 90 and 75%, respectively. Thepredicted service life of GFRP bars embedded in moist concreteat an isotherm temperature of 6°C to reach a PR of less than 75%can also be estimated to be infinite. These predictions show thatthe GFRP bars are a durable face to the concrete environment,which is supposed to be well simulated by the immersion of em-

Table 3. Results of Differential Scanning Calorimetry Analysis

ConditioningTemperature

�°C�Duration�days�

Tg Run 1�°C�

Tg Run 2�°C�

Unconditioned — — 105 134

Embedded in concreteand immersed in water

50 240 104 129

bedded GFRP bars in water. It is also evident that the traditional



environmental aging performed through immersion in simulatedconcrete pore solution is much harsher than the real concreteenvironment. Chen et al. �2006� found that the necessary time toreach a PR of 75% for similar GFRP bars, yet immersed in analkaline solution, was approximately 1.4 years at 20°C; the dura-tion is very low compared to more than 27 years for mortar-wrapped bars exposed to tap water at 23°C, as shown in Fig. 13.

Summary and Conclusions

Based on the results of this study the following conclusions maybe drawn on only the product tested:1. The aging of mortar-wrapped GFRP bars in tap water is sig-

nificantly less pronounced than traditional aging in simulatedpore solution. The loss in the tensile strength and themicrostructural/physical effects were significantly reduced;

2. Even at high temperature �50°C�, where the environment ismore aggressive, the change in tensile strength is minor. Forexample, increasing the temperature of the solution from40 to 50°C during 240 days results in a decrease of the ten-sile strength retention by 10–16% of the original tensilestrength;

3. No significant microstructural changes were observed after240 days immersion of GFRP bars embedded in concrete intap water at 50°C. The interfaces between the bars and con-crete and between the resin and the fibers do not seem to beaffect by the moisture absorption and high temperatures;

4. The polymer matrix is not affected by moisture absorptionand high temperatures: no changes of the glass transitiontemperature occur as observed by differential scanning calo-rimetry. FTIR did not show any significant changes of thepolymer chemical structure, i.e., degradation;

5. Long-term behavior predictions of conditioned GFRP barswere made by using a method based on Arrhenius relation.These predictions provide information about the long-termtensile strength retention. To be able to use Arrhenius rela-tion, we suppose that the mechanisms of degradation remainsthe same during the service life of the bars, but that they areaccelerated by aging; and

6. According to the long-term predictions, the tensile strengthretention of GFRP bars embedded in moist concrete will de-creased by 20 and 25% after 100 and 200 years, respectively.It was shown that the service lifetime allowed to reach ten-sile strength retention of less than 50% should be infinite.

Fig. 13. General relation between PR and predicted service life atmean annual temperature of 6°C �Montreal�

These results are in accordance with the findings of the

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analyses reported by Mufti et al. �2007�, who have tested fewstructures reinforced with GFRP after up to 9 years in serviceconditions.

Finally, this study clearly shows that the durability of testedmortar-wrapped bars, exposed to tap water, is less affected thanwhen exposed to simulated pore-water solution. It is reasonableto assume that the latter is too harsh in comparison to the realconcrete environment. The adverse consequence of such is theunderestimation of GFRP durability and conservative design ofconcrete structures reinforced by composite materials.

Acknowledgments

This research was supported by the National Science and Engi-neering Research Council �NSERC� of Canada and the CanadianNetwork of Centres of Excellence on Intelligent Sensing for In-novative Structures �ISIS Canada�.

References

ASTM. �2000�. “Standard test methods for density and specific gravity,�relative density� of plastics by displacement.” ASTM D 192,Philadelphia.

ASTM. �2003a�. “Standard test method for assignment of the glass tran-sition temperature by differential scanning calorimetry.” ASTM E1356, Philadelphia.

ASTM. �2003b�. “Standard test method for compositional analysis bythermogravimetry.” ASTM E 1131, Philadelphia.

ASTM. �2005�. “Standard specification for Portland cement.” ASTM C150, Philadelphia.

Bank, L. C., Gentry, T. R., Thompson, B. P., and Russel, J. S. �2003�.“A model specification for composites for civil engineering struc-tures.” Constr. Build. Mater., 17�6–7�, 405–437.

Benmokrane, B., and Wang, P. �2002�. “Durability of FRP compositesfor civil infrastructures applications, a state of art report.” TechnicalRep., Univ. of Sherbrooke, Sherbrooke, Qué., Canada.

Benmokrane, B., Wang, P., Ton-That, T., Rahman, H., and Robert, J.�2002�. “Durability of glass fibre reinforced polymer reinforcing barsin concrete environment.” J. Compos. Constr., 6�2�, 143–153.

Canadian Standards Association �CSA�. �2006�. “Canadian highwaybridge design code.” CAN/CSA-S6-06, Rexdale, Ont., Canada.

Chen, Y., Davalos, J. F., and Ray, I. �2006�. “Durability prediction forGFRP bars using short-term data of accelerated aging tests.” J. Com-pos. Constr., 10�4�, 279–286.

Chen, Y., Davalos, J. F., Ray, I., and Kim, H. Y. �2007�. “Acceleratedaging tests for evaluation of durability performance of FRP reinforc-ing bars reinforcing bars for concrete structures.” Compos. Struct.,78�1�, 101–111.

JOURNAL OF COM


Dejke, V. �1999�. “Durability of fibre reinforced polymers �FRP� as rein-forcement concrete structures.” Publication No. P-98:18, ChalmersUniv. of Technology, Göteborg, Sweden.

Dejke, V. �2001�. “Durability of FRP reinforcement in concrete-literaturereview and experiments.” Thesis, Degree of Licentiate of Engineer-ing, Chalmers Univ. of Technology, Göteborg, Sweden.

Diamond, S. �1981�. “Effects of two Danish flyashes on alkali contents ofpore solutions of cement fly ash pastes.” Cem. Concr. Res., 11, 383–394.

Gerritse, A. �1998�. “Assessment of long term performance of FRP barsin concrete structures.” Proc., Durability of Fiber Reinforced Poly-mers (FRP) Composites for Construction, B. Benmokrane and H.Rahman, eds., Sherbrooke, Qué., Canada, 285–297.

Katsuki, F., and Uomoto, T. �1995�. “Prediction of deterioration of FRProds due to alkali attack.” Proc., Non-Metallic (FRP) Reinforcementfor Concrete Structures, L. Taerwe, ed., Ghent, Belgium, 108–115.

Litherland, K. L., Okley, D. R., and Proctor, B. A. �1981�. “The use ofaccelerated aging procedures to predict the long term strength of GRCcomposites.” Cem. Concr. Res., 11, 455–466.

Micelli, F., and Nanni, A. �2004�. “Durability of FRP rods for concretestructures.” Constr. Build. Mater., 18�7�, 491–503.

Mufti, A. A., Banthia, N., Benmokrane, B., Boulfiza, M., and Newhook,J. P. �2007�. “Durability of GFRP composite rods.” Concr. Int., 29�2�,37–42.

Murphy, K., Zhang, S., and Karbhari, V. M. �1999�. “Effect of concretebased alkaline solutions on short term response of composites.” Proc.,44th Int. SAMPE Symp. and Exhibition, L. J. Cohen, J. L. Bauer, andW. E. Davis, eds., Society for the Advancement of Material and Pro-cess Engineering, Long Beach, Calif., 2222–2230.

Nelson, W. �1990�. Accelerated testing—Statistical models, test plans,and data analyses, Wiley, New York.

Nkurunziza, G. �2004�. “Performance de barres d’armature en polymèresrenforcés de fibre �PRF� de verre pour des structures en béton sous leseffets combinés de la charge soutenue, de la température et del’environnement humide et alcalin.” Ph.D. thesis, Univ. of Sher-brooke, Sherbrooke, Qué., Canada �in French�.

Porter, M. L., Mehus, J., Young, K. A., O’Neil, E. F., and Barnes, B. A.�1997�. “Aging for fiber reinforcement in concrete.” Proc., 3rd Int.Symp. on Non-Metallic (FRP) Reinforcement for Concrete Structures,Vol. 2, Japan Concrete Institute, Sapporo, Japan, 59–66.

Pultrall Inc. �2005� “FRP reinforcing bars data sheet.” �www.pultrall.com�.

Sen, R., Shahawy, M., Mullins, G., and Spain, J. �1999�. “Durability ofcarbon fiber reinforced polymer/epoxy/concrete bond in marine envi-ronment.” ACI Struct. J., 96�6�, 906–914.

Taylor, H. F. W. �1987�. “A method for predicting alkali ion concentrationin cement pore water solutions.” Adv. Cem. Res., 1�1�, 5–16.

Wang, P. �2005�. “Effect of moisture, temperature, and alkaline on dura-bility of E-glass/vinyl ester reinforcing bars.” Ph.D. thesis, Univ. ofSherbrooke, Sherbrooke, Qué., Canada.



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